Disk drives are digital data storage devices which may allow host computers to store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive may include a plurality of magnetic recording disks which are mounted to a rotatable hub of a spindle motor and rotated at a high speed. Information may be stored on each disk in concentric tracks. The data tracks are usually divided into sectors. Information may be written to and/or read from a storage surface(s) of a disk by a transducer or head. The transducer may include a read element separate from a write element, or the read and write elements may be integrated into a single read/write element. The transducer may be mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm may allow the transducer to access different data tracks. The disk may be rotated by the spindle motor at a relatively high speed, which may allow the transducer to access different sectors within each track on the disk.
The actuator arm may be coupled to a motor or coarse actuator, such as a voice coil motor (VCM), to move the actuator arm such that the transducer moves radially over the disk. Operation of the coarse actuator may be controlled by a servo control system. The servo control system generally performs two distinct functions: seek control and track following. The seek control function includes controllably moving the actuator arm such that the transducer is moved from an initial position to a target track position. In general, the seek function may be initiated when a host computer associated with the computer disk drive issues a seek command to read data from or write data to a target track on the disk.
As the transducer approaches the target track, the servo control system may initiate a settle mode to bring the transducer to rest over the target track within a selected settle threshold, such as a percentage of the track width from track center. Thereafter, the servo control system may enter the track following mode, where the transducer is maintained at a desired position with respect to the target track (e.g., over a centerline of the track) until desired data transfers are complete and another seek is performed.
The ability to precisely position a transducer with respect to a track being followed has become increasingly important as data and track densities in disk drives have increased. In particular, the space between adjacent tracks has become increasingly small, and the tracks themselves have become increasingly narrow. In order to increase the precision with which a transducer may be positioned with respect to a track during track following, an articulated actuator arm may be used. In general, the angle of the distal portion, or second stage, of the actuator arm with respect to the main portion, or first stage, of the actuator arm may be controlled by a micro actuator. The micro actuator may have a faster response than the coarse actuator to command signals, but may have a comparatively small range of movement. Thus, by operating the micro actuator to introduce small changes in the position of the transducer with respect to a track being followed, the accuracy of track following operations may be increased.
Because the location of the transducer is a combination of the contributions of the coarse actuator and the micro actuator, the position of the micro actuator within its relatively small range of motion typically may not be directly observable. Accordingly, the current position and response of the micro actuator to movement commands may be estimated through a model of the micro actuator. As such, the accuracy of the estimated response of the micro actuator to movement commands may substantially affect the precision with which the transducer can be positioned relative to a track.
For example, in some conventional techniques for estimating a response of the micro actuator, the coarse actuator may be used to maintain a desired position of the transducer relative to a target track, while a step function may be applied to the micro actuator. The resulting output position of the transducer responsive to the application of the step function to the micro actuator may be measured and used to calculate the gain of the micro actuator. A number of output positions may be measured to provide an average calibrated measurement. However, such techniques may not provide consistent results, as the coarse actuator may attempt to counteract the movement of the transducer caused by the response of the micro actuator, which may corrupt the measured output position. In addition, such techniques may be relatively slow, as it may be necessary to wait for the response of the coarse actuator to settle prior to the application of the next step function.